It is typical of machine guns and other automatic weapons that the firing mechanisms are complex and intricate structures. The large number of parts required increases the initial cost of production and raises the possibility of part failure. Weight is also a critical factor which may be increased by a complex firing mechanism.
Individual types of firing mechanisms incur problems common to that specific type of mechanism. Hammer actuated firing mechanisms, for example, are susceptible to misfires caused by light hammer strikes. This can be caused by a number of reasons. Excessive bolt carrier bounce can rob the hammer of sufficient energy and thus cause a light strike. A weak or broken hammer spring or foreign particles in the hammer mechanism could also be the cause of misfires which result from light hammer strikes. Inertia type firing mechanisms may incur similar problems. In this type of mechanism the operating rod, driven by the main action spring, impacts the firing pin to detonate a round. A weak or broken action spring, foreign matter anywhere in the mechanism, or poor lubrication might be the cause of a misfire in a system of this type. In all firing mechanisms the greater the number of parts, the lesser is its reliability and the greater is its replacement of worn or broken parts and time spent in maintaining the weapon. Hence, developing a reliable firing mechanism with a minimum of parts is always a challenge.